Shift device of hydraulic control system for automatic transmission

ABSTRACT

A hydraulic control system comprises first and second shift valves which cooperate with each other to effect a shift between two gear ratios. The first shift valve has a port and two different pressure acting areas which are exposed to pressure from the port depending upon downshift and upshift states of the first shift valve, respectively. The second shift valve effects communication between signal pressure generating means and the port of the first shift valve when the second shift valve is in one of two states to deliver the signal pressure to the port of the first shift valve.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydraulic control system for anautomatic transmission and more particularly to a shift device forcontrolling a shift between gear ratios in an automatic transmission.

2. Description of the Prior Art

The automatic transmission is designed such that gear ratios areselectively established by switching the friction element or elements,such as a clutch or a brake, to be engaged, and the control of a shiftis carried out by shift valves provided in a hydraulic control system.(For example, in the case of a forward three speed automatictransmission, the shift control is carried out by two shift valves,vis., a 1-2 shift valve and a 2-3 shift valve.) Commonly, a conventionalshift valve is constructed such that a throttle pressure indicative ofan opening degree of a throttle valve of an engine and a governorpressure indicative of a vehicle speed of a motor vehicle are exerted ona spool of the shift valve in opposed directions, allowing the spool toshift between two positions in response to the relationship in magnitudebetween them, thus supplying or discharging a line pressure to or from acorresponding friction element or elements. Therefore, a shift line,vis., a throttle opening degree vs., vehicle speed characteristic, onwhich the shift valve switches from one state to another state has beena straight line or a quadratic curve. With this shift line, it isdifficult to provide a drive feel fit for the characteristic of anengine and that of a motor vehicle over the whole throttle openingdegrees. For example, if shift points are set fit for operation within arelatively large throttle opening degree range, shift points become toohigh to be appropriate or too low to be appropriate for operation withina medium or small throttle opening degree range, thus failing to provideappropriate shift points. A conventional shift valve is provided with anoil pressure acting area on which an oil pressure (throttle pressure orline pressure or the like) acts when a spool is in a downshift position(for example, in the case of a 2-3 shift valve, the term "downshiftposition" is herein used to mean a position which the spool of the shiftvalve takes in a second gear ratio state and the term an "upshiftposition" is used to mean a position which the spool takes in a thirdgear ratio state) so as to provide a hysteresis between a shift pointfor an upshift and a shift point for a downshift but, with thisconstruction, it is difficult to provide a desired hysteresis at anygiven throttle opening degree over the whole throttle opening range andthus a shift point for an upshift may not be set in a desired manner orin an independent manner from the setting of a shift point for adownshift. In summary, since, in the case of the conventional shiftvalve, throttle pressure and governor pressure are compared with eachother by a single shift valve to determine a shift point, it is aproblem that since the shift points can not be chosen freely, a shiftpoint for an upshift and a shift point for a downshift can not be set ina desired manner over the whole throttle opening degree range.

SUMMARY OF THE INVENTION

According to the present invention, a hydraulic control system for anautomatic transmission comprises a first shift valve and a second shiftvalve wherein the first shift valve has a port and two differentpressure acting areas which are exposed to pressure from the portdepending upon downshift and upshift states of the first shift valve,respectively. The second shift valve effects communication betweensignal pressure generating means and the port of the first shift valvewhen the second shift valve is in one of two states thereof to deliverthe signal pressure to the port of the first shift valve.

Accordingly, an object of the present invention is to provide ahydraulic control system wherein it is easy and relatively unrestrictedto design a shift point for an upshift and a shift point for a downshiftover the whole range of operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described hereinafter in connection withthe accompanying drawings, wherein:

FIG. 1 is a schematic view of a change-speed gearbox used in anautomatic transmission;

FIG. 2 is a circuit diagram showing a hydraulic control system for theautomatic transmission according to the present invention;

FIG. 3 is a graph showing the characteristics of various pressuresdeveloped in the hydraulic control system of FIG. 2;

FIG. 4 is a graph showing, by way of example, the shift scheduleaccomplished by the hydraulic control system of FIG. 2;

FIG. 5 is a graph showing, by way of example, the simplifiedcharacteristics of two theoretical upshift lines U1 and U2 and twotheoretical downshift lines D1 and D2 provided by a 2-3 shift valve 4 ofthe hydraulic control system of FIG. 2;

FIG. 6 is graph showing, by way of example, the simplifiedcharacteristics of upshift and downshift lines Ux and Dx provided by a3-2 downshift valve 5 of the hydraulic control system of FIG. 2;

FIG. 7 is a graph showing, in bold solid line, an actual upshift lineand, in bold broken lines, an actual downshift line as obtained from thecharacteristics of the theoretical shift lines shown in FIGS. 5 and 6;

FIG. 8 is a graph showing, in bold solid line, another example of anactual upshift line and, in bold broken lines, another example of anactual downshift line as obtained from the characteristics of anotherexample of theoretical shift lines U1, U2, D1, D2, Ux and Dx shown inthis Figure;

FIG. 9 is a graph showing, in bold solid line, still another example ofan actual upshift line and, in bold broken lines, still another exampleof an actual downshift line as obtained from the characterstics of stillanother example of theoretical shift lines U1, U2, D1, D2, Ux and Dxshown in the Figure; and

FIG. 10 is a graph used to exaplain how the actual upshift and downshiftlines shown in FIG. 9 are obtained.

DESCRIPTION OF THE EMBODIMENT

Referring to FIG. 1, a change-speed gearbox of an automatic transmissionis illustrated which is controlled by a hydraulic control systemaccording to the present invention. The change-speed gearbox providesthree forward gear ratios and one reverse gear ratio and comprises acrank shaft 100 to be driven by an engine, a torque converter 101, aninput shaft 102, a front clutch 104 (a high and reverse clutch), a rearclutch 105 (a forward clutch), a second brake 106 (an intermediatebrake), a low-and-reverse brake 107, a one-way clutch 108, anintermediate shaft 109, a first planetary gear set 110, a secondplanetary gear set 111, an output shaft 112, a first governor valve 113,a second governor valve 114 and an oil pump 115.

The torque converter 101 comprises a pump impeller P, a turbine runner Tand a stator S, of which the pump impeller P is driven by the crankshaft 100 so that the torque converter working oil contained therein iscaused to swirl and imparts torque to the turbine runner T which issecured to the input shaft 102. The torque is further delivered throughthe input shaft 102 to the change-speed gearbox. The stator S is mountedabout a sleeve 116 with an one-way clutch 103 interposed therebetween.The one-way clutch 103 is constructed and arranged in such a manner asto permit a rotation of the stator S in the same direction as thedirection of rotation of the crank shaft 100, viz., the directionindicated by the arrow (abbreviated hereinafter as forward rotation) andto prevent the opposite rotation of the stator (abbreviated hereinafteras opposite rotation).

The first planetary gear set 110 comprises an internally toothed gear117 rotatable with the intermediate shaft 109, a sun gear 119 rotatablewith a hollow transmission shaft 118, two or more planet pinions 120,each meshing with the internally toothed gear 117 and the sun gear 119so that it rotates and moves along an orbit, and a front planet carrier121 rotatable with the output shaft 112 and having the planet pinions120 thereon; while the second planetary gear assembly 111 comprises aninternally toothed gear 122 rotatable with the output shaft 112, a sungear 123 rotatable with the hollow transmission shaft 118, two or moreplanet pinions 124, each meshing with the internally toothed gear 122and the sun gear 123 so that it rotates and moves along an orbit, and arear planet carrier 125 having the planet pinions 124.

Referring to the friction units, the front clutch 104 is operative toestablish a connection between the transmission input shaft 102 to bedriven by the turbine runner T and the hollow transmission shaft 118,rotatable in unison with the two sun gears 119 and 123 through a drum126, while the rear clutch 105 is operative to connect the input shaft102 and the internally toothed gear 117 of the first planetary gearassembly 110 through the intermediate shaft 109. The second brake 106 isoperative to tighten a band winding the drum 126 secured to the hollowtransmission shaft 118 so as to lock the two sun gears 119 and 123,while the low-and-reverse brake 107 is operative to lock the rear planetcarrier 125 of the second planetary gear assembly 111. On the otherhand, the one-way clutch 108 is so constructed and arranged as to permitthe forward rotation of the rear planet carrier 125 but prevent theopposite rotation of the same. The first governor valve 113 and secondgovernor valve 114 are fixed to the output shaft 112 and are operativeto produce a governor pressure indicative of the vehicle speed.

Description will be hereinafter made of the power flow paths which areestablished during operation in an automatic forward drive range (Drange).

Under this condition, the rear clutch 105 serving as the forward inputclutch is engaged. The power from the engine and having passed throughthe torque converter 101 is transmitted, through the input shaft 102 andrear clutch 105, to the internally toothed gear 117 of the firstplanetary gear set 110. The rotation of the internally toothed gear 117causes the planet pinions 120 for rotation in the forward direction.Since the sun gear 119 tends to rotate in the opposite direction to urgethe sun gear 123 of the second planetary gear set 111 rotatable with thesun gear 119 to rotate in the opposite direction, the planet pinions 124of the second planetary gear set 111 tend to rotate in the forwarddirection. The one-way clutch 108 is operative to prevent the rearplanet carrier 125 from tending to rotate in the opposite direction, sothat the sun gear 123 serves as a reaction brake in the forwarddirection. As a consequence, the internally toothed gear 122 of thesecond planetary gear set 111 rotates in the forward direction. Ittherefore follows that the output shaft 112 rotatable with theinternally toothed gear 122 also rotates in the forward direction,thereby producing the first forward gear ratio. When, under thiscondition, the second brake 106 is applied after the vehicle speed hasincreased, the power which has passed through the input shaft 102 andthe rear clutch 105 as in the first gear condition is transmitted to theinternally toothed gear 117. The second brake 106 is operative to lockthe drum 126 to prevent rotation of the sun gear 119, thus serving as areaction brake in the forward direction. Accordingly, the planet pinions120 rotate and move along an orbit around the sun gear 119 which is heldstationary with the result that the front planet carrier 121 and thetransmission output shaft 112 integral with the former rotate in theforward direction at a speed although with a reduction ratio higher thanthe speed which would be achieved under the first gear ratio condition,thereby producing the second forward gear ratio. When the second brake106 is released and the front clutch 104 is engaged after the vehiclespeed has increased further, the power delivered to the input shaft 102splits into a portion transmitted through the rear clutch 105 to theinternally toothed gear 117 and into the remaining portion transmittedthrough the front clutch 104 to the sun gear 119. Therefore, theinternally toothed gear 117 and the sun gear 119 are interlocked torotate together with the front planet carrier 121 and the output shaft112 at a common revolution speed in the forward direction, therebyproducing the third forward gear ratio. Under this condition, the frontclutch 104 and the rear clutch 105 may be referred to as an input clutchand there is no reaction brake so that the planetary gear sets do notlend themselves to multiplication of torque.

The power flow path to be established for a reverse drive range (Rrange) will be hereinafter described.

When this range is selected, both of the front clutch 104 andlow-and-reverse brake 107 are made operative. The power from the enginehaving passed through the torque converter 101 is transmitted from theinput shaft 102 through the front clutch 104 and the drum 126 to the sungears 119 and 123. Since, under this condition, the rear planet carrier125 is locked by the low-and-reverse brake 107, the rotation of the sungears 119 and 123 in the forward direction causes the internally toothedgear 122 to rotate at a reduced speed in the reverse direction with theresult that the output shaft 112 rotatable with the internally toothedgear 122 rotates in the reverse direction, thereby producing the reversedrive gear ratio.

The change-speed gearbox is controlled by the hydraulic control systemshown in FIG. 2.

Referring to FIG. 2, the hydraulic control system comprises a regulatorvalve 1, a manual valve 2, a 1-2 shift valve 3, a 2-3 shift valve 4, a3-2 downshift valve 5, a line pressure booster valve 6, a pressuremodifier valve 7, a throttle valve 8, a throttle failsafe valve 9, athrottle modulator valve 10, a first gear range pressure reducing valve11, an accumulator 12, a 3-2 timing valve 14 and a front clutch pressurereducing valve 15, all these valves being connected as shown in theillustrated circuit network to the torque converter 101, the frontclutch 104, the rear clutch 105, a band servo 106' for operating theabove described second brake 106 (see FIG. 1), the low-and-reverse brake107, the governor valves 113 and 114 and the oil pump 115. The shiftdevice according to the present invention comprises as a first shiftvalve a 2-3 shift valve 4 and as a second shift valve a 3-2 downshiftvalve.

The oil pump 115 is driven by the engine through the crank shaft 100 andthe pump impeller P of the torque converter 101 and is operative to suckin an oil from an oil reservoir, not shown, the oil cleared of harmfuldust by means of an oil strainer (not shown) and to feed the oil to aline pressure circuit 16 when the engine is in operation. The regulatorvalve 1 which is adapted to regulate the pressure of the oil to apredetermined level comprises a valve spool 1b, which is urged by meansof a spring 1a for movement toward a raised position indicated by theleft half of the spool in the drawing, slidably mounted within a housing1c and also comprises four chambers 1d, 1e, 1f and 1g. To each of thechambers 1d and 1f is fed an oil pressure from the line pressure circuit16 by way of oil conduits 17 and 18. To the chamber 1e is fed a linepressure from the port 2b of the manual valve 2 through an oil conduit22 when the manual valve 2 is in any one of the D range, II range and Irange positions which will be described later. The valve spool 1b has aland 1b' having a diameter slightly smaller than the diameter of thecorresponding rib 1c' of the housing 1c so as to form therebetween asmall clearance which serves as a variable area orifice. The oil in thechamber 1f is constantly discharged through this clearance and a drainport 1h at a rate which is determined by an amount of overlap betweenthe land 1b' and the rib 1c' so that a high line pressure proportionalto the amount of overlap is developed in the line pressure circuit 16.The valve spool 1b further has a land 1b" which is slightly smaller indiameter than the bore 1c" in the housing 1c so as to form a smallclearance therebetween so that the oil in the chamber 1f is suppliedthrough this clearance and an oil conduit 19 to the torque converter101, the oil cooler 20 and the various lubricating parts 21 in thetransmission mechanism.

The line pressure developed in the line pressure circuit 16 is directedto the manual valve 2, which serves as a fluid-flow directionchange-over valve adapted to provide communication from the linepressure circuit 16 selectively to any one of the ports 2a, 2b, 2c, 2dand 2e when the selector lever (not shown) is manipulated for rangeselection, the valve comprising a valve spool 2f which is slidablymounted within a housing 2e. The valve spool 2f is movable between theneutral (N) range position, an automatic forward drive (D) rangeposition, a manual second (II) range position, a manual first (I) rangeposition, a reverse drive (R) range position and a parking (P) rangeposition and is constructed such that the line pressure circuit 16communicates with the ports indicated by the sign "o" in the followingtable when the above mentioned selector lever is operated to urge thevalve spool 2f to move to these range positions. The ports which are notin communication with the line pressure circuit 16 are all made open tothe drain openings 2h' and 2h" on both of the housing 2e and thus serveas drain ports.

    ______________________________________                                        Ports                                                                         Range   2a         2b    2c       2d  2g                                      ______________________________________                                        R       o                             o                                       N                                     o                                       D                  o                  o                                       II                 o     o                                                    I                  o     o        o                                           ______________________________________                                    

The first governor valve 113 and the second governor valve 114 areoperative to develop a governor pressure having a magnitudecorresponding to vehicle speed under forward drive ranges of a vehicle.When the manual valve 2 is in any one of the ranges D, II and I, theline pressure is first fed to the second governor valve 114 from theport 2b communicating with the line pressure circuit 16 by way of theconduit 22 as will be understood from the above table and, when thevehicle is moving, the second governor valve 114 regulates the linepressure into a governor pressure indicative of the vehicle speed andfeeds this governor pressure to the first governor valve 113, where,when the vehicle speed increases beyond a predetermined value, the firstgovernor valve 113 starts to allow the governor pressure into a governorpressure circuit 23. The governor pressure is thereafter distributed viathe circuit 23 to the 1-2 shift valve 3, 2-3 shift valve 4 and 3-2downshift valve 5 and controls the operations of these valves in themanners to be described later.

The 1-2 shift valve 3 comprises a housing 3a and two valve spools 3b and3c which are arranged axially in line with each other and which areslidably mounted within the housing 3a. That end face of the valve spool3b which is remoter from the valve spool 3c is acted upon by a spring 3dand that end face of the valve spool 3c which is remoter from the valvespool 3b is located in a chamber 3e. The valve spool 3b is formed withlands 3f, 3g, 3h and 3i which are larger in diameter in this sequence,while the housing 3a if formed with bores 3i, 3j, 3k and 3l whichcorrespond to these lands, respectively. The spool 3c is formed withlands 3n, 3o and further with lands 3p, 3q which are larger in diameterthan the former two lands, while, the housing 3a is formed with twobores 3r, 3s for the land 3n and a bore 3t for the land 3o. The 1-2shiftvalve 3 is in communication with the governor pressure circuit 23, akickdown pressure circuit 24 and a throttle modulator pressure circuit25 as shown in the drawing and further with an oil conduit 27 which isto communicate with an oil conduit 26 or a drain port 3s depending uponthe axial position of the land 31. The governor pressure circuit 23communicates with the chamber 3e, while, the kickdown pressure circuit24 acts upon a differential area between the lands 3f, 3h when the spool3b is in the position indicated by the right half thereof and acts uponthe differential area between the lands 3g, 3i when the spool 3b is inthe position indicated by the left half thereof. The throttle modulatorpressure circuit 25 acts upon a differential area between the lands 3h,3i when the spool 3b is in the position indicated by the right halfthereof and is prevented from acting on the same when the spool 3b is inthe position indicated by the left half thereof. The oil conduit 26 isin communication with an output port 28a of a shuttle valve 28, and theoil conduit 27 is in communication with the low-and-reverse brake 107.The 1-2 shift valve 3 is further in communication with an oil conduit 30extending from an oil conduit 29 which leads to the rear clutch 105after branching from the oil conduit 22 that leads from the port 2b tothe governor valves 113, 114, and between the 1-2 shift valve 3 and 2-3shift valve 4 is arranged an oil conduit 31 as connected to the 1-2shift valve such that it is allowed to communicate with the oil conduit30 or is prevented from communicating with same depending upon axialposition of the land 3p. The oil conduit 31 communicates with a drainport 3t when the valve spool 3c is in the position indicated by theright half thereof. In the oil conduit 29 is provided an orifice 74 anda check valve 77 which are arranged in parallel with each other. A port3w of a 1-2 shift valve 3 communicates via an oil conduit 96 with an oilconduit 59 that communicates with the port 2d of the manual valve 2 suchthat the oil pressure (line pressure from the port 2d of the manualvalve 2) acts upon a differential area between the lands 3p, 3c when thespool 3c is in the position indicated by the right half thereof.

The 2-3 shift valve 4 comprises a housing 4a with a wall portiondefining a valve bore and a valve element in the form of a valve spool4b. The spool 4b is movable within the valve bore of the housing 4abetween a downshift position, indicated by the right half in thedrawing, and an upshift position, indicated by the left half in thedrawing. The upper end face of the spool 4b is acted upon by a spring 4dwithin a chamber 4s, while, the lower end face thereof communicates witha chamber 4e. The spool 4b is formed with different diameter lands 4g,4h, 4i and 4j which are larger in diameter in this sequence, and a land4k of the same diameter as that of the land 4j, and a land 41 larger indiameter than this land, while, the housing 4a is formed with differentdiameter bore sections 4m, 4n, 4p, 4 q and 4r for these lands. The lands4g, 4h, 4i, 4j, 4k and 4l are axially spaced from each other. The spool4b has a circumferential groove between each two adjacent lands of thelands 4g, 4h, 4i and 4j. An oil conduit 32 is connected to a linepressure outlet port of the 2-3 shift valve such that it is allowed tocommunicate with an oil conduit 99 via a line pressure inlet port of the2-3 shift valve and is prevented from communicating with a drain port 4wor is prevented from communicating with the oil conduit 99 and isallowed to communicate with the drain port 4w depending upon the axialposition of the land 4j. The land 4j closes the line pressure inlet portwhen the spool 4b is in the downshift position to, in turn, blockcommunication between the oil conduits 99 and 32. The land 4j opens theline pressure inlet port when the spool 4b is in the upshift positionto, in turn, effect communication between the oil conduits 99 and 32.The oil conduit 99 communicates with the port 2g of the manual valve 2and is provided with an orifice 98 and a check valve 97 which arearranged in parallel. The chamber 4e communicates with the governorpressure circuit 23 to receive governor pressure, while, the chamber 4sreceiving the spring 4d is always drained. The governor pressure circuit23 leads also to the port 4t so that the governor pressure acts upon adifferential area between the lands 4l, 4k when the spool 4b is in thedownshift position indicated by the right half. A conduit 36 which isconnected to a 3-2 downshift valve 5 communicates with the port 4x suchthat a throttle modulator pressure, which will be described later, actsupon a differential area (S1) between the lands 4i, 4g when the spool 4bis in the downshift position indicated by the right half in the drawingor upon a differential area (S3) between the lands 4j, 4h when the spool4b is in the upshift position indicated by the left half in the drawing.A throttle modulator pressure circuit 25 communicates, via an oilconduit 38, with the port 4u of the 2-3 shift valve 4 such that it actsupon a differential area (S2) between the lands 4j, 4i when the spool 4bis in the downshift position indicated by the right half in the drawing.The port 4u is closed by the land 4j when the spool 4b is in the upshiftposition. A differential area (S4) between the lands 4k, 4l is exposedto the port 4t when the spool 4b is in the downshift position. The port4t is closed by the land 4l when the spool 4b is in the upshiftposition.

The 3-2 downshift valve 5 comprises a housing 5a with a wall portiondefining a different diameter bore and a valve element in the form of avalve spool 5b. The valve spool 5b is movable within the valve bore ofthe housing 5a between a down-side position indicated by the right halfin the drawing and an up-side position indicated by the left half in thedrawing. Acted on one end face of the spool 5b is a spring 5c, while,the other end face is exposed to a chamber 5d. The oil conduit 36 isconnected to a signal pressure outlet port of the 3-2 downshift valve 5such that depending upon the axial position of the land 5e of the spool5b it communicates with a signal pressure inlet port to which an oilconduit 39 extending from the throttle modulator pressure circuit 25 isconnected or a kickdown pressure port 5f, while, the chamber 5dcommunicates with the governor pressure circuit 23. The port 5f isconnected to a kickdown pressure circuit 24, while, a chamber 5greceiving the spring 5c is always drained. The lands 5h, 5e and 5i aresmaller in diameter in this sequence. The lands 5h, 5e and 5i areaxially spaced from each other and the spool 5b has a circumferentialgroove between each two adjacent lands.

The line pressure booster valve 6 comprises a housing 6a having a valvespool 6b slidably mounted therein, the valve spool 6b being urged tomove leftwardly in the drawing by means of a spring 6c. The valve spool6b is formed with grooves 6d and 6e and an oil conduit 6g for providingcommunication between the groove 6e and a chamber 6f. The line pressurebooster valve 6 is in communication with an oil conduit 40 to be open tothe groove 6e when the valve spool 6b has moved to the left and an oilconduit 41 to be open to the groove 6e when the valve spool 6b has movedto the right. The oil conduit 40 which is joined by the oil conduit 32extends to an orifice 69, a check valve 68, a 3-2 timing valve 14 and afront clutch valve 15, while, the oil conduit 41 connects with the oilconduit 31 and merging into an oil conduit 42 leading for connection toa servo apply chamber 106'a of the band servo 106'. The line pressurebooster valve 6 is further in communication with an oil conduit 43 whichis always open to the groove 6d and with an oil conduit 45 which is tobe selectively brought into communication with the oil conduit 43through the groove 6d depending upon the axial position of the valvespool 6b. The oil conduit 43 is in communication with one input port 46aof a shuttle valve 46 and the oil conduit 45 is in communication withthe port 2c of the manual valve 2c.

The transmission throttle valve 8 comprises a housing 8a having a valvespool 8b slidably mounted therein and a plunger 8d provided in line withthe valve spool 8b via a spring 8c. The plunger 8d is connected to theaccelerator pedal by means of, for example, a mechanical linkage and isadapted to be moved rightwardly in the drawing from an idling positionindicated by the upper half of the plunger and thereby adding to theforce of the spring 8c when the accelerator pedal is depressed. Thevalve spool 8b is formed with a groove 8e and a throttle pressurecircuit 48 and an oil conduit 49 are provided in communication with thethrottle valve 8 in such a manner as to be always open to the groove 8e.The throttle valve 8 is further provided with a drain port 8f whichcommunicates with the throttle pressure circuit 48 through the groove 8edepending upon the axial position of the valve spool 8b, and with an oilconduit 50 extending from the line pressure circuit 16, while, the oilconduit 49 is in communication with a chamber 8g. Increasing the forceof the spring 8 by moving the plunger 8d rightwardly as the acceleratorpedal is depressed causes a throttle pressure to be provided to thethrottle pressure circuit 48, the throttle pressure being created withinthe chamber 8g by draining the line pressure to the drain port 8f so asto balance with the spring force. Thus, the throttle valve 8 delivers athrottle pressure which corresponds to the force of the spring 8c (viz.,the distance of stroke of the accelerator pedal depressed or throttleopening degree) and which is proportional to the throttle opening degreein a pattern as shown by a-e-b shown in FIG. 3. When the acceleratorpedal is depressed to a kickdown position, the plunger 8d compresses thespring 8c to a full extent until is is brought into abutting engagementwith the valve spool 8b and forces the valve spool 8b to move to a limitposition closing the drain port 8f and thereby providing communicationbetween the throttle pressure circuit 48 and the oil conduit 50. Underthese conditions, the throttle pressure increases to a magnitude equalto that of the line pressure.

The throttle failsafe valve 9 comprises a sleeve 9a which is slidablymounted within the same housing 8a in such a manner as to guide theplunger 8d, the leftward movement of the sleeve being elasticallylimited by means of a spring 9b. The coil conduit 47 providingcommunication between the other inlet port 46b of the shuttle valve 46and the throttle failsafe valve 9 communicates normally with a port 9cof the throttle failsafe valve 9. The throttle pressure circuit 48communicates with a chamber 9d having accommodated therein the spring 9bon one hand, and on the other hand communicates via the port 9c with achamber 9f to which an enlarged portion 8j of the plunger 8d is exposed,while, the kickdown pressure circuit 24 communicates with a port 9g. Tothe throttle failsafe valve 9 an oil conduit 52 leads from the linepressure circuit 16, which oil conduit is normally closed, but under anabnormal condition when the sleeve 9a takes the lower half position inthe drawing, the oil conduit 52 is allowed to communicate with the oilconduit 47. During the movement of the plunger 8d in such a direction asbeing depressed, the throttle pressure developed in the throttlepressure circuit 48 is directed via the port 9e to the chamber 9f andacts on the enlarged portion 8j of the plunger 8d and imparts to theplunger 8d a force effective to move the plunger 8d inwardly against theforce of the spring 8c, thus preventing the accelerator pedal from beingexcessively loaded by the spring 8c as the pedal is depressed. When,furthermore, the plunger 8d is moved into a kickdown position, thekickdown pressure circuit 24 which has been in communication with thedrain port 8h through the port 9g is isolated from the drain port 8h andis permitted to communicate with the oil conduit 48 through the port 9e,chamber 9f and port 9g. Under this condition, the valve spool 8b ismoved rightwardly in the drawing as previously described and, as aconsequence, the line pressure in the oil conduit 50 is passed into thethrottle pressure circuit 48 without being drained off, therebydeveloping in the circuit 24 a kickdown pressure which is equal inmagnitude to the line pressure. The kickdown pressure thus delivered issupplied also to the throttle modulator valve 10 by way of an oilconduit 53. If a damage should take place in the mechanical linkageinterconnecting the accelerator pedal and the plunger 8d and the plunger8d should disengage from the accelerator pedal, the plunger 8d would beurged by a return spring, not shown, to move to the idling positionwhich is indicated by the upper half of the plunger, and this movementof the plunger 8d forces the sleeve 9a to move leftwardly into theposition indicated by the lower half thereof. In this state, since noforce is imparted to the valve spool 8b by the spring 8c, the valvespool 8b assumes a position allowing the drain port 8f to be slightlyopened and substantially fully closing the oil conduit 50. The oilconduit 47 is allowed to communicate with the oil conduit 52 in thisstate, thus delivering the line pressure to the oil conduit 47. The linepressure in the oil conduit 47 reaches through the shuttle valve 46 apressure modifier valve 7 where it is modulated to a magnitude balancingwith that spring force of a spring 7c when a valve spool 7b is in thelefthand position as indicated in the drawing and the thus modulatedpressure is supplied through an oil conduit 54 to the chamber 1g of thepressure regulator valve 1, boosting the line pressure to its maximumvalue. As a consequence, the friction elements are actuated by the linepressure with the maximum value so as to enable the vehicle to reach arepair shop without any assistance thereto and without causing a burntdamage due to a slip in the friction elements.

The pressure modifier valve 7 comprises a housing 7a having a valvespool 7b slidably mounted within the housing and having one end faceacted upon by a spring 7c and the other end face exposed to a chamber7d. The valve spool 7b is formed with a groove 7e, while the housing 7ais formed with an output port 7f constantly open to this groove, a drainport 7g and an input port 7h. The ports 7g and 7h are arranged in such amanner that one of the ports is on the point of being opened when theother of the ports is on the point of being closed during movement ofthe valve spool 7b, the port 7f communicating with the chamber 7dthrough the oil conduit 54 on one hand and with the chamber 1g of theregulator valve 1, the port 7h communicating with the outlet port 7h ofthe shuttle valve 46.

With the pressure modifier valve 7 thus constructed, the spring 7c holdsthe valve spool 7b in any position which is lower than the positionindicated by the left half in the drawing when the oil pressuredeveloped in the port 7h is smaller than a set force of the spring 7c(the set force of the spring 7c being a spring force as achieved whenthe valve spool 7b is held in the position indicated by the left halfthereof), thereby closing the drain port 7g and providing communicationbetween the port 7f and the port 7h so that the oil pressure developedin the port 7h is directed to the port 7f and further through the oilconduit 54 to the regulator valve 1. Throughout these conditions, theoil pressure is directed also into the chamber 7d and causes the valvespool 7b to move from the position indicated by the right half thereofto the position indicated by the left half thereof against the force ofthe spring 7c as the oil pressure increases. If, however, the oilpressure to be delivered from the port 7f tends to further increase, thevalve spool 7b is moved upwardly beyond the position indicated by theleft half of the spool and permits the port 7f to communicate with thedrain port 7g with the result that the oil pressure to be delivered intothe oil conduit 54 is prevented from increasing beyond a certain valuewhich is dictated by the force of the spring 7c as provided under acondition in which the valve spool 7b is held in the position indicatedby the left half thereof, so that when the throttle pressure in thecircuit 48 is supplied via the shuttle valve 46 to the port 7h, thepressure modifier valve 7 produces an oil pressure called a pressuremodifier pressure which varies such that the oil pressure stopsincreasing after the throttle opening degree has increased furtherbeyond, for example, two fourths (2/4) of the full opening degree asshown by a-e-f in FIG. 3.

The throttle modulator valve 10 comprises a housing 10a having slidablymounted therein a valve spool 10e which is formed with three lands 10b,10c and 10d (lands 10b, 10c having the same diameter, while, the land10d with a diameter smaller than the former two) and which has one endface thereof being acted upon by a spring 10g which spring force isadjustable by means of an adjuster 10f and the other end face thereofexposed to a chamber 10h. A circuit 25 is connected to the housing 10ain such a manner as to be at all times open to the groove between thelands 10b and 10c, while, an oil conduit 53 and an oil conduit 56 (viz.,an oil conduit branching from the line pressure circuit 16) areconnected to the housing 10a in such a manner that one of these oilconduits is initiated to open when the other thereof completely closesduring movement of the valve spool 10e. The housing 10a is furtherconnected with an oil conduit 57 aligned with the connecting port of thecircuit 25, the oil conduit 57 being in communication with a chamber 10ihaving a spring 10g accommodated therein. Furthermore, a chamber 10h isprovided which is in comminication with the throttle valve 8 through anoil conduit 49.

With the throttle modulator valve 10 thus constructed, the valve spool10e is held in the position indicated by the lower half thereof by theforce of the spring 10g when the throttle pressure directed through theoil conduit 49 into the chamber 10h is zero. Under this condition, theoil conduit 56 extending from the line pressure circuit 16 is isolatedfrom the circuit 25 and the oil conduit 57 by means of the valve spool10b and the circuit 25 and the oil conduit 57 are held in communicationwith the drain port 8h through the oil conduit 53 and the port 9g of thethrottle failsafe valve 9 so that there develops no oil pressure in thecircuit 25 and the oil conduit 57. As the throttle pressure rises, thevalve spool 10e moves beyond the position indicated by the upper halfthereof against the force of the spring 10g to direct the line pressurewithin the oil conduit 56 to the chamber 10i through the oil conduit 57,urging in cooperation with the spring 10g back to the balanced positionindicated by the upper half in the drawing. The throttle modulator valve10 is thus capable of regulating the line pressure from the oil conduit56 in response to the throttle pressure directed into the chamber 10hand delivers a throttle modulator pressure to the throttle modulatorpressure circuit 25, the throttle modulator pressure varies inproportion to the throttle opening degree as shown by g-h in FIG. 3.Under kickdown conditions in which the plunger 8d of the throttle valve8 is urged to move inwardly, the port 9g is isolated from the drain port8h as previously discussed and, as a consequence, the throttle pressure(b-c-d in FIG. 3) corresponding to the line pressure is fed from theport 9g to the throttle modulator valve 10 by way of the oil conduit 53so that an oil pressure corresponding to the line pressure is developedin the throttle modulator pressure circuit 25 and the oil conduit 57 andthis pressure is directed into the chamber 10i for urging the valvespool 10e to move to the leftward limit position thereof with theresult, under kickdown condition, an oil pressure corresponding inmagnitude to the line pressure as shown by h-c-d in FIG. 3 is alwaysdeveloped in the circuit 25.

The manual first range pressure reducing valve 11 comprises a housing11a having slidably mounted therein a valve spool 11b which has one endface acted upon by a spring 11c and the other end face exposed to achamber 11d. The valve spool 11b is formed with a groove 11e and an oilconduit 58 is permitted to constantly open to this groove, and the oilconduit 58 is allowed to communicate with one input port 28b of theshuttle valve 28 and with the chamber 11d. The housing 11a is furtherprovided with a drain port 11f and is connected to an oil conduit 59extending from the port 2d of the manual valve 2, these drain port 11fand oil conduit 59 being arranged in such a manner that one of themstarts to open when the other completely closes during movement of thevalve spool 11b.

Thus, the manual first range pressure reducing valve 11 is operative topartially discharge the line pressure directed from the manual valve 2to the oil conduit 59 and reduces the line pressure to a constant leveldetermined by the force of the spring 11c in a condition indicated bythe left half thereof when the I range is selected, the pressure thusobtained being delivered to the oil conduit 58 so that thelow-and-reverse brake 107 which is to be also operative under reversedrive condition is precluded from producing an excessively large torquetransmission capacity.

The port 2a of the manual valve 2 is connected through an oil conduit 60to the other input port 28c of the shuttle valve 28 and is alsoconnected to an input port 61a of a shuttle valve 61 whose output port61b is allowed to communicate through an oil conduit 62 with the frontclutch 104. Provided in the oil conduit 60 is a parallel combination ofan orifice 78 and a check valve 79. An accumulator 12 comprises astepped piston 12b and a stepped cylinder 12c fitted thereto to definechambers 12a, 12d and 12e, and a spring 12f urging the piston 12bdownwardly as viewed in the drawing. The chamber 12d communicatesthrough an oil conduit 63 with the oil conduit 29, and the chamber 12ecommunicates through an oil conduit 64 with the oil conduit 42. Thechamber 12a is drained. Provided in the oil conduit 42 upstream of theaccumulator 12 is a parallel combination of an orifice 75 and a checkvalve 76.

The 3-2 timing valve 14 comprises a housing 14a having slidably mountedtherein a valve spool 14b which has one end face acted upon by a spring13c and the other end face exposed to a chamber 14d. The valve spool 14bis responsive to the governor pressure directed from the governorpressure circuit 23 to the chamber 14d by way of an oil conduit 66 andis movable into a lower position, as indicated by the right half of thevalve spool 15b in the drawing, allowing the oil conduit 40 to open toan oil conduit 67 leading to the servo release chamber 106'b of the bandservo 106' and a raised position isolating the oil conduit 65 from theoil conduit 67 as indicated by the left half of the spool 14b. Betweenthe oil conduits 40 and 67 is provided a parallel combination 80 of acheck valve 68 and an orifice 69 which bypasses the 3-2timing valve 14.

The front clutch pressure reducing valve 15 comprises a housing 15ahaving slidably mounted therein a valve spool 15b which has one end faceacted upon by a spring 15c and the other end face exposed to a chamber15d. The valve spool 15b is formed with a groove 15e forming lands onboth sides thereof. The housing 15a is provided with an oil conduit 70which is constantly open to the groove 15e and which is in communicationwith the other input port 61c of the shuttle valve 61. The housing 15ais formed also with a drain port 15g and is in communication with theoil conduit 40, the drain port and the oil conduit being arranged sothat one of them is permitted to open when the other of them iscompletely closed during movement of the valve spool 15b. The housing15a is still further formed with an oil conduit 71 which is open inradial alignment with the oil conduit 70 and which is in communicationwith the chamber 15d, and a chamber 15h receiving the spring 15c thereinis drained.

With the front clutch pressure reducing valve 15 thus constructed, thevalve spool 15b is normally urged by the spring 15c to a lower positionisolating the oil conduit 70 from the drain port 15g and connected tothe oil conduit 40 as indicated by the right half of the valve spool.When, therefore, the line pressure is directed to the oil conduit 40through the oil conduit 32 under the control of the 2-3 shift valve 4,this line pressure is directed through the oil conduit 70 and theshuttle valve 61 and the oil conduit 62 to the front clutch 104. Thispressure is directed also to the chamber 15d through the oil conduit 71to urge the spool 15b upwardly in the drawing. The spring 15c acts uponthe spool 15b to urge same downwardly in the drawing to the balancedposition of the valve spool 15b where the downwardly directed urgingforce balances with the upward directed urging force. When the pressuresupplied to the front clutch 104 reaches a predetermined value, thespool 15b is urged upwardly to the position indicated by the left halfin the drawing to isolate the oil conduit 70 from the oil conduit 40 andto allow the former to communicate with the drain port 15g, causing thevalve spool to take a balanced position as shown in the left half in thedrawing. Therefore, pressure to be supplied to the front clutch 104 doesnot increase beyond a predetermined level.

The operation of the hydraulic control system is hereinafter explained.

In the first place, the pressure regulator valve 1 is supplied with thepump pressure directed to the chamber 1d from the oil pump 115, themodifier valve pressure directed to the chamber 1j from the pressuremodifier valve 7 and the line pressure directed to the chamber 1e fromthe port 2b of the manual valve 2 only when the manual valve 2 is set toD range or II range or I range, urging the valve spool 1b for downwardmovement in the drawing. Into the chamber 1g is directed the modifiervalve pressure which, in cooperation with the force of the spring 1a,urges the valve spool 1b for upward movement in the drawing. The valvespool 1b is held in a position in which the forces thus exerted thereonare balanced, thereby developing in the circuit 16 a line pressure whichis determined by such a position of the valve spool (FIG. 3, m-n-d). Theline pressure thus developed is always directed through the circuit 16into the port 2h of the manual valve 2. When P range or R range or Nrange is selected, the chamber 1e of the pressure regulator valve 1 isdrained off through the port 2b of the manual valve 2 with the resultthat the line pressure to be developed in the circuit 16 under suchconditions (FIG. 3, p-g-r) is made higher than that achieved under anyone of the D range or II range or I range conditions because of the factthat there is no force effective to urge the valve spool 1b to movedownwardly in the absence of the line pressure in the chamber 1e of thepressure regulator valve 1.

The line pressure supplied to the throttle modulator valve 10 via theline pressure circuit 16 and oil conduit 56 is modulated by thisthrottle modulator valve 10 into said throttle modulator pressure to bedelivered into the circuit 25.

When the driver of the vehicle urges the manual valve 2 to move from Nrange to D range, the port 2h communicates with the ports 2b, 2g so thatthe line pressure is directed from the port 2b through the oil conduits22, 29 to the rear clutch 105 and also directed from the port 2g throughthe oil conduit 99 to the 2-3 shift valve 4. The line pressure passingthrough the oil conduit 29 is restricted by the orifice 74 on its way tothe rear clutch 105 and is fed to the rear clutch 105 initially at alimited rate and thereafter at a gradually increasing rate. This rearclutch supply pressure is also directed through the oil conduit 63 tothe accumulator chamber 12d to urge the stepped piston 12b for downwardmovement toward a larger diameter side. With the engagement of the rearclutch 105, the automatic transmission enables the vehicle to start torun with the first gear ratio. The line pressure thus supplied from theport 2g of the manual valve 2 through the oil conduit 99 is stopped bythe 2-3 shift valve under this condition, causing no function.

The line pressure passed through the port 2b of the manual valve 2 tothe oil conduit 22 is also directed to the governor valves 113 and 114to cause the delivery to the circuit 23 a governor pressure indicativeof the vehicle speed as previously described. Since the port 2b of themanual valve 2 communicates with the line pressure circuit 16 to allowthe line pressure to extend to the oil conduit 22 throughout all of theforward range positions (D), (II) and (I), the governor pressure isdelivered to the governor pressure circuit 23 whenever the manual valve2 is set in the above mentioned forward drive range.

When the vehicle speed reaches a certain value after the vehicle hasmoved off from the rest, the governor pressure indicative of the vehiclespeed and directed to the chamber 3e of the 1-2 shift valve 3 overcomesthe downward force with which the spring 3d exerts on the valve spools3b and 3c in the positions indicated by the right halves of the spoolsand the downward force with which the modulated throttle pressuredirected from the circuit 25 acts on the differential pressure actingarea between the lands 3h and 3i, the valve spools 3b and 3c begin tomove upwardly from the positions indicated by the right halves thereof.During this upward movement, when the land 3h disengages out of the bore31, the chamber between the lands 3h, 3i is drained off through thekickdown pressure circuit 24 via the drain port 8h of the throttle valve8, causing the downward force which has been exerted upon the spool 3cdue to the application on the differential area between the lands 3h, 3iof the throttle modulator pressure to disappear, thus causing the spools3b, 3c to move instantaneously into the positions indicated by the lefthalves of the spools in the drawing. This allows the oil conduit 30branching off the oil conduit 29 to communicate with the oil conduit 31,thus allowing the line pressure which has been directed to the oilconduit 29 as having been described to extend through the oil conduit 30and 1-2 shift valve 3 to the oil conduit 31. The line pressure to be fedthereafter to the servo apply chamber 106'a of the band servo 106'through the oil conduit 42 is restricted by the orifice 75 on its waytoward the servo apply chamber. The servo apply pressure is directedthrough the oil conduit 64 also to the accumulator chamber 12e to urgethe stepped piston 12b in the downward position as previously describedto move back against the spring 12f and the pressure within the chamber12d. This causes the servo apply pressure to rise slowly and enables theband servo 106' to actuate the second brake 106 slowly. This actuationof the second brake 106 causes an upshift from the first gear ratio tothe second gear ratio in the transmission in cooperation with thepreviously described engagement of the rear clutch 105 and a shock whichis experienced upon this upshift can be alleviated by the abovementioned operation of the accumulator 12.

As the vehicle speed further increases during operation of the vehiclewith the second gear ratio, the governor pressure indicative of thisvehicle speed which has reached through the circuit 23 to the chamber 4eof the 2-3 shift valve 4 urges the spool 4b in the downshift positionindicated by the right half in the drawing upwardly from this positionovercoming the total of a downward force due to the spring 4d and adownward force created by throttle modulator pressure from the port 4uacting upon differential area (S2) between the lands 4i, 4j. During thisupward movement, when the land 4i disengages from the bore 4p, thechamber between the lands 4i, 4j begins to communicate with the oilconduit 36. The oil conduit 36 is drained off through the 3-2 downshiftvalve 5 as it is in the up-side position and kickdown pressure circuit24 to the drain port 8h of the throttle valve 8. The land 4j closes theport 4u. These cause the downward force due to the above-mentionedthrottle modulator pressure to disappear, thus allowing the spool 4b tomove upwardly instantaneously to the upshift position indicated by theleft half in the drawing. This effects communication of the oil conduit99 with the oil conduit 32, allowing the line pressure having beendirected to the oil conduit 99 as previously described to be deliveredthrough the 2-3 shift valve 4 and oil conduit 32 to the oil conduit 40.This line pressure from the oil conduit 40 reaches via the passages 6e,6g of the line pressure booster valve 6 to the chamber 6f, urging thespool 6f for movement in the rightward direction viewing in the drawingfrom the position indicated by the upper half toward the positionindicated by the lower half, and the line pressure is directed thoughthe orifice 69 and check valve 68 to the servo release chamber 106'b andthe front clutch pressure reducing valve 15. The line pressure directedto the front clutch reducing valve 15 is reduced according to thepreviously mentioned pressure modulation function of this valve, and thefront clutch 104 is supplied with the thus reduced pressure through theoil conduit 70, shuttle valve 61 and oil conduit 62. The application ofthe line pressure to the servo release chamber 106b' urges the piston ofthe band servo 106' to move back toward the servo apply chamber 106a'because the above mentioned piston has a larger pressure acting area onthe side adjacent to the servo release chamber 106'b than that on theservo apply chamber 106'a. This causes the release of the second brake106 by means of the band servo 106'. The release of the second brake 106during engagement of the front clutch 104 causes an upshift from thesecond gear ratio to the third gear ratio in the automatic transmission.

Under a condition wherein the spool 5b of the 3-2 downshift valve 5 isin the up-side position indicated by the left half in the drawingagainst the spring 5c owing to the force built up by the governorpressure fed to the chamber 5d of the 3-2 downshift valve 5 from thecircuit 23, if the throttle opening is increased above a predetermineddegree as a result of the depression of the accelerator pedal, thethrottle modulator pressure indicative of this throttle opening degreeis directed from the throttle modulator pressure circuit 25 via thecircuit 39 to the 3-2 downshift valve 5 to act upon a differential areabetween the lands 5e, 5h and urge the spool 5b in cooperation with thespring 5c for downward movement to the down-side position indicated bythe right half in the drawing. Communication is now provided between theoil conduits 36 and 39 so that the modulated throttle pressure is fedthrough the oil conduits 39 and 36 to the chamber 4v of the 2-3 shiftvalve 4 via the port 4x and acts upon a differential area (S3) betweenthe lands 4h, 4j of the spool 4b, causing the valve spool 4c fordownward movement from the position indicated by the left half to theposition indicated by the right half thereof against the force resultingfrom the governor pressure developed in the chamber 4e. This blockscommunication between the oil conduits 99 and 32 so that the supply ofthe line pressure to the oil conduit 32 is interrupted and at the sametime the oil conduit 32 is permitted to communicate with the drain port4w, whereby the oil pressure which has been fed to the front clutch 104and the servo release chamber 106'b under the third gear ratio conditionis discharged in a manner to be described in the following. As the oilpressure in the chamber 15d of the front clutch pressure reducing valve15 is eliminated and as a consequence the valve spool 15b is urged tomove by the force of the spring 15c to the position providingcommunication between the oil conduits 40 and 70 as indicated by theright half of the valve spool, the front clutch pressure is dischargedat a comparatively high rate through the oil conduit 62, the shuttlevalve 61, the oil conduits 70, 40 and 32 and the drain port 4w. On theother hand, the servo release chamber is drained off through the oilconduit 67, oil conduit 80, orifice 69, oil conduit 40 to the drain port4w at a comparatively slow rate owing to the provision of the orifice69. As the vehicle speed has lowered, the governor pressure indicativeof the vehicle speed and directed from the governor pressure circuit 23through the oil conduit 66 to act upon the chamber 14d of the 3-2 timingvalve 14 cannot hold the spool 14b in the position indicated by the lefthalf in the drawing against the force of the spring 14c and thus thespool 14 moves down to the position indicated by the right half in thedrawing wherein communication between the oil conduits 40 and 67 isestablished. In this case, the servo release pressure is dischargedthrough the oil conduit 67, the 3-2 timing valve 14, the oil conduit 40and the drain port 4w (bypassing the orifice 69), at a rate which iscomparatively higher than the rate to be achieved when the vehicle speedis high. In these manners, the servo release pressure is discharged at arelatively low rate, when compared with the discharge rate of the frontclutch pressure, determined by the flow rate of oil through the orifice69 under high vehicle speed conditions and at a relatively high ratewhen the vehicle speed is relatively low. For these reasons, theactuation of the band servo 106' (and accordingly of the second brake106 as well) as compared with the timing at which the front clutch 104is uncoupled is retarded at high vehicle speeds, thereby making itpossible to achieve a prolonged neutral interval during which the engineis enabled to increase its output speed to a value to match the vehiclespeed while a downshift is being made from the third gear ratio to thesecond gear ratio without producing gear shift shocks. At low vehiclespeeds, the amount of retardation in the actuation of the second brake106 as compared with the timing at which the front clutch 104 is to beuncoupled is reduced so that the amount of retardation provides theperiod of time which is required for the engine to increase itsrevolution speed to a level to match the vehicle speed, thereby makingit possible to reduce the gear shift shocks to be produced during theabove described downshift.

In the same manner as described as to the case when the throttle openingdegree is high, the 3-2 downshift valve 5 causes a downshift from thethird gear ratio to the second gear ratio in the automatic transmissionas long as the throttle opening degree is higher than a certain valueeven if as a consequence of a drop in the vehicle speed, the governorpressure directed to the chamber 5d also drops accordingly. However, inthe case when the throttle opening degree is lower than the certainvalue, the 3-2 downshift takes place irrespective of the operation ofthe 3-2 downshift valve 5. Explaining this, if the vehicle speed dropswith the throttle opening degree lower than the certain value, beforethe 3-2 downshift valve 5 is switched to the down-side position as aresult of a drop in the governor pressure, the downward force with whichthe spring 4d biases the spool 4b downwardly overcomes the upward forcewith which the spool 4b is urged upwardly by the governor pressure, thusswitching the 2-3 shift valve 4 into the downshift position indicated bythe right half in the drawing. This switching action of the 2-3 shiftvalve 4 causes the 3-2 downshift in a previously described manner. Theswitching action of the 2-3 shift valve 4 under this condition iscarried out in response to the relationship between the force of thespring 4d and the governor pressure and not in response to the throttlemodulator pressure, thus allowing a downshift to take place always at afixed vehicle speed. Therefore, a 3-2 downshift line which is bent isobtained as shown in FIG. 4.

As the vehicle speed further decreases, the governor pressure in thechamber 3e of the 1-2 shift valve 3 becomes no longer capable ofovercoming the force of the spring 3d so that the spring causes thevalve spools 3b and 3c to move downwardly from the positions indicatedby the left halves to the positions indicated by the right halves in thedrawing, thereby isolating the oil conduits 30 and 31 from each otherand permitting the oil conduit 31 to communicate with the drain port 3v.As a consequence, the line pressure which has been fed to the servoapply chamber 106a' is discharged passing through the check valve 76 inthe oil conduit 42, through the oil conduit 31 and the drain port 3v,thereby causing the band servo 106' to release the second brake 106.Among the friction elements, the rear clutch 105 only is in theoperative state so that a downshift is effected from the second gearratio to the first gear ratio in the automatic transmission.

When the manual valve 2 is thereafter urged to move back to N range, theport 2b is drained so that the line pressure which has been fed to therear clutch 105 is discharged through the oil conduit 29, the checkvalve 77 and the oil conduit 22 and further by way of the port 2b of themanual valve 2 and renders all the friction elements of the automatictransmission inoperative, establishing a neutral state in which thetransmission of power is interrupted. The line pressure in the port 2greaches the 2-3 shift valve 4 but the line pressure is blocked by theland 4j of the spool 4b so that there occurs no actuation of thefriction elements.

When the accelerator pedal is fully depressed to produce a kickdowncondition while the vehicle is running with the above described thirdgear ratio, the plunger 8d of the throttle valve 8 is urged to move intothe rightward limit position thereof as previously described so that akickdown pressure (having the magnitude of line pressure) is developedin the circuit 24. The kickdown pressure thus developed is directed onone hand to the port 3x of the 1-2 shift valve 3 and on the other handto the port 5f of the 3-2 downshift valve 5. Since, under kickdowncondition at high vehicle speeds, the spool 5b of the 3-2 downshiftvalve 5 is in the up-side position wherein the kickdown port 5f isallowed to communicate with the port 5j, the kickdown pressure directedto the port 5f of the 3-2 downshift valve 4 is fed to the port 4x of the2-3 shift valve 4 through the oil conduit 36. The kickdown pressuresupplied to the port 4x acts upon differential pressure acting area (S3)between the lands 4j and 4h of the spool 4b which is in the upshiftposition indicated by left half in the drawing and cooperates with thespring 4d to urge the spool 4b for downward movement to the downshiftposition indicated by the right half in the drawing. As a consequence,the 2-3 shift valve 4 effects a downshift from the third gear ratio tothe second gear ratio in the automatic transmission in a manner similarto that which has been described. If the vehicle speed decreasesfurther, the kickdown pressure which is fed from the circuit 24 to theport 3x of the 1-2 shift valve 3 acts on the differential area betweenthe lands 3i and 3g, cooperating with the spring 3d to urge the spools3b, 3c for downward movement from the position indicated by the positionindicated by the left half in the drawing to the position indicated bythe right half in the drawing against the governor pressure actingwithin the chamber 3e. As a consequence, the 1-2 shift valve 3 effects adownshift from the second gear ratio to the first gear ratio in a mannersimilar to that which has been described. When, in the kickdown state,the downshift valve 3 and 2-3 shift valve 4 are in the positionsindicated by the right half positions thereof in the drawing, the linepressure provided to the circuit 25 acts upon the differential areabetween the lands 3h, 3i of the spool 3b of the 1-2 shift valve 3 whichis in the position indicated by the right half in the drawing and alsoupon the differential area between the lands 4i and 4j of the spool 4bof the 2- 3 shift valve 4 which is in the position indicated by theright half in the drawing, thus urging the spools downwardly. Thekickdown pressure in the circuit 24 acts upon the differential areabetween the lands 3f and 3h of the spool 3b which is in the positionindicated by the right half in the drawing and also upon thedifferential area between the lands 4g and 4i of the spool 4b which isin the downshift position indicated by the right half in the drawing,urging the respective spools downwardly. Besides, the downward forcesdue to the springs 3d and 4d act upon the spools of both shift valves 3and 4. The above described downward forces thus exerted on the spools ofthe shift valves 3 and 4 are opposed by the forces resulting from thegovernor pressure developed in the chambers 3e and 4e so that, when thevehicle speed becomes such a value that the governor pressure resultingtherefrom overcomes the downward force exerted on the valve spool of the1-2 shift valve, then the 1-2 shift valve 3 effects an upshift from thefirst gear ratio to the second gear ratio in the manner previouslydescribed and, when the vehicle speed becomes such a value that thegovernor pressure resulting therefrom overcomes the downward forceexerted on the valve spool of the 2-3 shift valve 4, then the 2-3 shiftvalve 4 effects an upshift from the second gear ratio to the third gearratio in the manner previously described. Since, however, the downwardforces exerted on the valve spools of shift valves 3 and 4 are strongerthan the downward forces which are exerted thereon under ordinarythrottle valve opening degree range, an upshift can not be effected andaccordingly acceleration can be achieved by a large driving power with alow gear ratio before the vehicle speed is increased beyond levelscorresponding to the ordinary throttle valve opening degree range. Theshift line diagram obtained as explained above is illustrated in FIG. 4.

The operation is explained when II range is selected by the manual valve2 under a condition in which the vehicle is running with the third gearratio with the manual valve 2 held in the D range. Under a condition inwhich the third gear ratio in D range is in play, the line pressuredirected to the oil conduit 40 is passed through the groove 6e and theoil passage 6g to the chamber 6f and causes the valve spool 6b to movefrom the position indicated by the upper half to the position indicatedby the lower half thereof against the force of the spring 6c. After thismovement, the valve spool 6b is maintained in the latter position by theline pressure which is directed from the oil conduit 31 to the chamber6f by way of the groove 6e and the oil passage 6g. When the manual valve2 is moved to the II range position thereof under these conditions, theline pressure circuit 16 is permitted to communicate with the ports 2band 2c, while, the port 2g is drained off. The line pressure is directedthrough the port 2b to the same places as those previously described andthrough the port 2c to the oil conduit 45. If the port 2g is drained,the oil conduit 32 is drained off through the 2-3 shift valve 4 and oilconduit 99, and the oil conduit 32 is left drained off irrespective ofthe positions of the spool 4b of the 2-3 shift valve 4, withdrawing thepressure which has been supplied to the front clutch 104 and the servorelease chamber 106'b to render the automatic transmission to downshiftfrom the third gear ratio to the second gear ratio. Therefore, anupshift to the third gear ratio will not take place even if the vehiclespeed rises again. Since the line pressure booster valve 6 is in theabove-mentioned state, the line pressure directed to the oil conduit 45is directed to the port 7h of the pressure modifier valve 7 through theoil conduit 43 and the shuttle valve 46. This causes the throttlemodifier valve 7 to deliver into the oil conduit 54 an upper limit value(e-f in FIG. 3) of the pressure modifier pressure due to the pressuremodulation carried out by the pressure modifier valve 7 and thussupplies this modifier pressure with this limit value to the regulatorvalve 1 over the whole throttle opening degrees. As a result, theregulator valve 1 provides in the line pressure circuit 16 a linepressure with an upper limit magnitude (n-d in FIG. 3) over the wholethrottle opening degrees. For this reason, a sufficiently high linepressure can be produced and accordingly the rear clutch 105 and theband servo 106' are actuated powerfully during low to medium throttlevalve opening degrees, thereby ensuring sufficiently effective enginebraking under the II range condition.

When the vehicle speed decreases down to a certain value while thevehicle is running under the II range condition, the valve spool 3b ofthe 1-2 shift valve 3 is urged to move downwardly from the positionindicated by the left half to the position indicated by the right halfof the spool under the influence of the spring 3d with the result that adownshift is effected from the second gear ratio to the first gear ratioin the automatic transmission in a manner similar to that previouslydescribed. Under the conditions, the oil pressure which has beendeveloped in the oil conduit 31 is eliminated and, as a consequence, thevalve spool 6b of the line pressure booster valve 6 is liberated from aforce holding the valve spool 6b in the righthand position indicated bythe lower half of the spool and is therefore urged to move back to theposition indicated by the upper half of the spool by the force of thespring 6c. As a result, the line pressure in the oil conduit 45 isprevented from extending beyond the line pressure booster valve 6 andthe oil conduit 43 is permitted to communicate with the drain port 9c ofthe throttle failsafe valve 9. To the port 7h of the pressure modifiervalve 7 is thus supplied the line pressure from the oil conduit 48 bythe switching action of the shuttle valve 46, thereby enabling thepressure modifier valve 7 to feed the modified pressure to the pressureregulator valve 1 by way of the oil conduit 54 as previously describedso that the pressure regulator valve 1 is made operative to develop theabove mentioned line pressure in the circuit 16.

As the vehicle speed increases thereafter and as a consequence thegovernor pressure developed in the chamber 3e of the 1-2 shift valve 3renders the shift valve into the upshift state thereof, an upshift iseffected from the first gear ratio to the second gear ratio in theautomatic transmission in the manner previously described. When the linepressure developed in the oil conduit 31 is directed via the oil conduit41 to the line pressure booster valve 6 under this condition, the valvespool 6b of the valve is maintained in the position indicated by theupper half of the spool and is not caused to move rightwardly.Accordingly, upon shifting into the second gear ratio when the II rangeis selected under the condition wherein the vehicle is moving with thethird gear ratio or when the I range is selected under the conditionwherein the vehicle is moving with the third gear ratio, the latter casebeing discussed later, the line pressure is maintained at a constantlevel which is relatively high throughout the whole range of thethrottle valve opening degrees as previously described and enables thesecond brake to grasp the clutch drum with a sufficient force so as toensure engine braking under II range condition. Once the first gearratio is achieved, however, the line pressure can not be boosted andaccordingly the gear shift shock is not amplified even when repeatedalternate downshift and upshift are made between the second gear ratioand the first gear ratio and vice versa. When a shift is to be made fromthe second gear ratio in D range to II range or I range, the secondbrake is kept on grasping the clutch drum so that the line pressureneeds not be augmented by means of the line pressure booster valve 6.

When the manual valve 2 is then urged to move to I range, the linepressure circuit 16 is permitted to communicate not only with the ports2b and 2c but with the port 2d. The line pressure passed through theports 2b and 2c is directed to the same places as those previouslydescribed and the line pressure passed through the port 2d is suppliedto the manual first range pressure reducing valve 11. There beinginitially no oil pressure in the chamber 11d of the pressure reducingvalve 11, the valve spool 11b is maintained in the lower positionindicated by the right half of the spool by the force of the spring 11c.When, however, the line pressure directed from the oil conduit 59 ispassed to the chamber 11d and causes the valve spool 11b to moveupwardly so that the line pressure is partially discharged through thedrain port 11f, the valve spool assumes a balanced position indicated bythe left half of the spool, thereby reducing the line pressure to avalue equal to the force of the spring 11c acting on the valve spool inthe particular position. The line pressure directed to the oil conduit59 is reduced to a constant and the reduced pressure thus obtained ispassed through the oil conduit 58, the shuttle valve 28 and the oilconduit 26 and acts on the land 3n of the spool 3c of the 1-2 shiftvalve 3, exerting a downward force on the valve spool 3c. At a vehiclespeed at which the downward force is smaller than the upward forceresulting from the governor pressure developed in the chamber 3e, thevalve spools 3b and 3c are held in the positions indicated by therespective left halves of the spools and maintain the automatictransmission in the second gear ratio, thereby preventing the enginefrom overrunning which would otherwise be caused when, for example, Irange is selected while the vehicle is running at a high speed. In thisinstance, the line pressure is augmented by means of the line pressurebooster valve 6 to assure effective engine braking in the same manner aspreviously described in connection with the operation under II rangeposition only when the second gear ratio is established as a result ofselecting I range position during operation of the vehicle with thethird gear ratio. As the vehicle speed decreases and as a consequencethe upward force resulting from the governor pressure developed in thechamber 3e decreases, the valve spool 3c moves downwardly to theposition indicated by the right half of the spool by the downward forceresulting from the previously mentioned constant and reduced oilpressure acting on the land 3l of the valve spool 3c. On the other hand,the spool 3b is maintained in the position indicated by the left halfthereof and spaced apart from the spool 3c by the above mentionedconstant reduced oil pressure with the spring 3d in a compressed state.Under this condition, the oil conduit 27 which has been opened to thedrain port 3s is permitted to communicate with the oil conduit 26 andallows the constant reduced oil pressure in the oil conduit 26 to bedirected through the oil conduit 27 to the low-and-reverse brake 107 sothat the automatic transmission can drive the vehicle under I rangecondition while producing an engine braking effect by means of thelow-and-reverse brake 107 thus actuated and the rear clutch 105 which iskept engaged. The manual first range pressure reducing valve 11 isadapted to reduce the line pressure from the oil conduit 59 to aconstant value dictated by the force of the spring 11c and delivers thereduced oil pressure to the oil conduit 58 so that the shift point forthe 1-2 shift valve 3 can be chosen to occur at a desired constantvehicle speed under manual first condition for thereby preventing,without any delay, the engine from overrunning over the whole the rangeof the throttle valve opening degree.

When the manual valve 2 is urged to move from N range to R range, theline pressure circuit 16 is permitted to be in communication with theports 2a and 2g. From the port 2a, the line pressure is passed throughthe oil conduit 60 and is directed on one hand through the shuttle valve28 and the oil conduit 26 to the 1-2 shift valve 3 and further throughthe oil conduit 27 to the low-and-reverse brake 107 via the 1-2 shiftvalve because the valve spools 3b and 3c are held in the positionsindicated by the right halves thereof in the absence, in the chamber 3e,of a governor pressure which is to be developed only under forward drivegear condition. On the other hand, the line pressure is supplied throughthe orifice 78, shuttle valve 61 and oil conduit 62 to the front clutch104. The line pressure to be passed to the front clutch 104 isrestricted by the orifice 78 on its way to the front clutch so that theoil pressure to be developed in the front clutch increases initially ata low rate and thereafter at a gradually increasing rate. Therefore, theautomatic transmission mechanism is thus made operative to drive thevehicle rearwardly with the front clutch 104 coupled and the low andreverse brake 107 actuated. It may be noted that the line pressure atthe port 2g of the manual valve 2 is blocked by the 2-3 shift valve 4 inthe same manner as in N range.

When the manual valve 2 is urged to move back to N range, the port 2a isallowed to open to the drain port so that the line pressure in the frontclutch 104 is discharged quickly through the oil conduit 62, the shuttlevalve 61, oil conduit 60, check valve 79 and port 2a of the manual valve2, while, the line pressure in the low-and-reverse brake 107 isdischarged quickly by way of the oil conduits 27, 26, shuttle valve 28,oil conduit 60 and port 2a of the manual valve 2, thereby rendering theautomatic transmission into the neutral condition.

Hereinafter, more precise description as to the 2-3 shift valve 4 and3-2 downshift valve 5 follows. The 2-3 shift valve 4 cooperates with the3-2 downshift valve 5 to provide a 2-3 upshift line and a 3-2 downshiftline as shown in FIG. 4, but, these shift lines can be changed widely asexplained hereinafter if a cross sectional area of each land of the 2-3shift valve and 3-2 downshift valve 5 and the specification of thesprings 4d, 5c are changed.

Firstly, consideration is made of a state not under kickdown conditionviz., a case wherein kickdown pressure does not exist in the kickdowncircuit 24. In the 2-3 shift valve 4, an upwardly directed force,viewing in FIG. 2, is derived from the governor pressure acting on thelower end face of the land 41 only, but, a downwardly directed force isthe total of forces derived from the oil pressure acting on differentialarea S1 between the lands 4g and 4i when the spool 4b is in thedownshift position indicated by the right half position thereof, the oilpressure acting on differential area S2 between the lands 4i and 4j whenthe spool 4b is in the downshift position, and the oil pressure actingon differential area S4 between the lands 4k and 4l when the spool 4b isin the downshift position indicated by the right half position inaddition to the downwardly directed force due to the spring 4d, and whenthe spool 4b is in the upshift position as indicated by the left half inthe drawing, a downwardly directed force is derived in addition to theforce of the spring 4d from the oil pressure acting on differential areaS3 between the lands 4h and 4j. However, the oil pressure does notalways act on the differential areas S1 and S3, but, the throttlemodulator pressure is fed through the oil conduit 36 to act on thedifferential areas S1 and S3 only when the 3-2 downshift valve 5 is inthe down-side position and these areas are drained when the 3-2downshift valve 5 is in the up-side position. Oil pressures acting uponthe differential areas S1 through S4 are tabulated depending upon theabove-mentioned two cases as follows:

    ______________________________________                                                    2-3 Shift Valve                                                                                               shift                             3-2 Downshift Valve    S1     S2   S4  S3   line                              ______________________________________                                        up-side       upshift  X      X    X   X    D1                                position      position                                                                      downshift                                                                              X      Pm   P.sub.G                                                                           X    U1                                              position                                                        down-side     upshift  X      X    X   Pm   D2                                position      position                                                                      downshift                                                                              Pm     Pm   P.sub.G                                                                           X    U2                                              position                                                        ______________________________________                                         (In the Table, X designates drain, Pm throttle modulator pressure, P.sub.     governor pressure.)                                                      

As will be clear from this Table, the 2-3 shift valve 4 has an upshiftline and a downshift line depending upon each of the two states taken bythe 3-2 downshift valve 5. It therefore has two upshift lines U1 and U2and two downshift lines D1 and D2. These shift lines are shown in FIG.5. Since in usual cases the governor pressure P_(G) traces a quadraticcurve, the upshift lines U1 and U2 take also quadratic curves in thesecases, but for the sake of simplicity they are drawn as straight lines.The inclination and position of each of the shift lines may be changedin response to variation in the dimension of each land, thecharacteristic of the spring 4d of the 2-3 shift valve 4. These fourshift lines are theoretical shift lines and therefore not all of thempractically appear, but the shift is carried out along one of the U1 andU2 and one of the D1 and D2 depending upon the state of the 3-2downshift valve 5. These shift lines practically used are hereinaftercalled as actual shift lines. The position of the 3-2 downshift valve 5is determined upon the relationship between the governor pressure andthrottle modulator pressure. When the spool 5b is in the down-sideposition indicated by the right half thereof, the upshift line Ux of the3-2 downshift valve 5 is determined by the balance of an upwardlydirected force, viewing in the Figure, derived from the governorpressure acting on the lower end face of the land 5h and downwardlydirected force derived from the throttle modulator pressure acting onthe differential area between the lands 5h and 5i in addition to theforce of the spring 5c. When the spool 5b is in the up-side position asindicated by the left half position thereof, a downshift line Dx isdetermined by the balance of the downwardly directed force derived fromthe throttle modulator pressure acting on differential area between thelands 5e and 5h in addition to the downwardly directed force by thespring 5b and upwardly directed force derived from the governor pressureacting on the lower end face of the land 5h. These shift lines Ux and Dxare shown in FIG. 6. The inclination and position of these shift linesmay be changed in response to variation in the dimension in each of thelands and the characteristic of the spring 5c of the 3-2 downshift valve5.

Actual upshift lines and actual downshift lines shown by the solid andbroken bold lines in FIG. 7 through 9, respectively, may be obtained bychanging each of the shift lines U1, U2, D1 and D2 of the 2-3 shiftvalve 4 and changing each of the shift lines Ux and Dx of the 3-2downshift valve 5. Of course, the other many combinations are possible.As previously described, the actual shift lines along each of which theautomatic transmission is subject to a shift are determined by any oneof the combinations of the shift lines U1, U2, D1 and D2 and the shiftlines Ux and Dx, and therefore in the cases of the combinations as shownin the above-mentioned FIGS. 7 through 9 the actual shift lines as shownby the bold lines are obtained.

The way how the actual shift lines shown by the bold lines in FIG. 9 aredrawn is explained based upon FIG. 10. With the throttle opening degreeis maintained to the degree of 3/4, if the vehicle speed increasesgradually along the line A, both of the 2-3 shift valve 4 is in thedownshift position and 3-2 downshift valve 5 is in the down-sideposition, thus providing the second gear ratio. Since the 3-2 downshiftvalve 5 is maintained in the down-side position until line A intersectsthe line Ux and the 2-3 shift valve 4 is subject to a shift change alongthe shift lines U2 and D2, the 2-3 shift valve 4 is subject to anupshift at the point a where the shift line A intersects the line U2. Itgoes the same from a point b to a point c, thus providing the actualshift line as shown by the bold line section L1. If the vehicle speedincreases along a line B with the throttle opening degree 3/8, the 3-2downshift valve 5 is maintained in the down-side position until the lineB intersects the shift line Ux at a point d. However, the line B doesnot intersect with the shift line U2 before it intersects Ux, thus the2-3 shift valve 4 is maintained in the downshift state. If the line Bintersects the shift line Ux at the point d, the 3-2 downshift valve 5changes into the up-side position, causing the 2-3 shift valve 4 toeffect a shift along the shift lines U1 and D1, but, since the point dis within a range beyond the shift line U1, the 2-3 shift valve 4changes into the upshift state immediately, causing an upshift to takeplace. Viz., the 2-3 shift valve 4 changes into the upshift state at thesame time when the 3-2 downshift valve 5 changes into the up-sidepositio. It goes the same from the point c to the point e (theintersections with the lines U1 and Ux), thus providing the bold linesection L2. If the vehicle speed gradually increases along the line Cwith the throttle opening degree below 1/4, the 3-2 downshift valve 5changes into the up-side position when the line C intersects the shiftline Ux. Thus, the 2-3 shift valve 4 is caused to change along the shiftlines U1 and D₁, causing an upshift at point f when the line Cintersects the shift line U1. It goes the same from the point e to thepoint g, thus providing the bold line section L3. Accordingly, theactual bent shift line as shown by the bold line sections L1, L2 and L3is obtained. In the same manner, the actual downshift line as shown bythe bold line may be obtained.

Under kickdown, the line pressure prevails in the oil conduit 36irrespective of the position of the 3-2 downshift valve 5 as previouslymentioned, two sets of shift lines do not exist with respect to the 2-3shift valve 4.

In the preceding description, the present invention has been applied to2-3 shift, but it may be applied to 1-2 shift or 3-4 shift of a fourspeed automatic transmission.

What is claimed is:
 1. A hydraulic control system for an automatictransmission for a motor vehicle having an internal combustion enginewith a throttle valve, the automatic transmission including at least oneforward drive gear ratio lower than at least one other forward drivegear ratio, the transmission including a plurality of friction elementsincluding a friction element for contributing to shifting between theone forward drive gear ratio and the one other forward drive gear ratio,the hydraulic control system comprising:a first shift valve having ahousing with a wall portion defining a valve bore; a second shift valvehaving a housing with a wall portion defining a valve bore; linepressure generating means for delivering a line pressure to said firstshift valve; first signal pressure generating means for delivering afirst signal pressure to said first and second shift valves; secondsignal pressure generating means for delivering a second signal pressureto said first and second shift valves; said first shift valve includinga line pressure inlet port, a line pressure outlet port, a first signalpressure port and a second signal pressure port receiving said secondsignal pressure, said line pressure inlet and outlet ports and saidfirst signal pressure and second signal pressure ports being disposed insaid wall portion of said first shift valve; said line pressure inletand outlet ports being selectively communicating with each other; saidsecond shift valve including a signal pressure inlet port receiving saidsecond signal pressure, and a signal pressure outlet prior communicatingwith said first signal pressure port of said first shift valve, saidsignal pressure inlet and signal pressure outlet ports being disposed insaid wall portion of said second shift valve; said signal pressure inletand outlet ports of said second shift valve selectively communicatingwith each other; said first shift valve including a valve elementmovable within said valve bore of said first shift valve between adownshift position and an upshift position; said valve element of saidfirst shift valve including means for effecting communication betweensaid line pressure inlet and outlet ports when said valve element ofsaid first shift valve is in the upshift position to, in turn, effectcommunication between said line pressure generating means and thefriction element to establish the one forward drive gear ratio, saidvalve element of said first shift valve also including means forblocking communication between said line pressure inlet and outlet portswhen said valve element is in the downshift position to, in turn, blockcommunication between said line pressure generating means and thefriction element; said valve element of said first shift valve includinga first pressure acting area exposed to pressure from said first signalpressure port of said first shift valve when said valve element of saidfirst shift valve is in the downshift position, a second pressure actingarea exposed to second signal pressure from said second signal pressureport of said first shift valve when said valve element of said firstshift valve is in the downshift position, and a third pressure actingarea exposed to pressure from said first signal pressure port of saidfirst shift valve when said valve element of said first shift valve isin the upshift position, said valve element of said first shift valvehaving an end pressure acting area exposed to said first signal pressurefrom said first signal pressure generating means when said valve elementof said first shift valve is in any one of the downshift and upshiftpositions, said second shift valve including a valve element movable insaid valve bore of said second shift valve between first and secondpositions; said valve element of said second shift valve including meansfor effecting communication between said signal pressure inlet andsignal pressure outlet ports of said second shift valve when said valveelement of said second shift valve is in the first position to, in turn,effect communication between said second signal pressure generatingmeans and said first signal pressure port of said first shift valve,said valve element of said second shift valve further including meansfor blocking communication between said signal pressure inlet and signalpressure outlet ports of said second shift valve when said valve elementof said second shift valve is in the second position to, in turn, blockcommunication between said second signal pressure generating means andsaid first signal pressure port of said first shift valve; said valveelement of said second shift valve further including a first pressureacting area exposed to second signal pressure from said signal pressureinlet port of said second shift valve when said valve element of saidsecond shift valve is in the first position and a second pressure actingarea exposed to second signal pressure from said signal pressure inletport when said valve element of said second shift valve is in the secondposition, said valve element of said second shift valve having an endpressure acting area exposed to said first signal pressure from saidfirst signal pressure generating means.
 2. A hydraulic control system asclaimed in claim 1, wherein said first signal pressure is a governorpressure indicative of a vehicle speed of the motor vehicle, and saidsecond signal pressure is a throttle modulator pressure indicative of anopening degree of the throttle valve.
 3. A hydraulic control system asclaimed in claim 1 or 2, wherein said valve element of said first shiftvalve is a valve spool having thereon first, second, third and fourthcircumferential lands which are larger in diameter in this sequence andwhich are axially spaced from each other, said valve element of saidfirst shift valve including at least one circumferential groove betweeneach two adjacent lands, said first pressure acting area of said valveelement of said first shift valve includes a circumferential groovebetween said first and second circumferential lands and acircumferential groove between said second and third circumferentiallands, said second pressure acting area includes said groove betweensaid third and fourth circumferential lands, said third pressure actingarea includes the groove between said second and third circumferentiallands and the groove between said third and fourth circumferentiallands, and said fourth land blocks said second signal pressure port whensaid valve element of said first shift valve is in the upshift position.4. A hydraulic control system as claimed in claim 3, wherein said valvespool includes a fifth land, said valve element of said first shiftvalve including a circumferential groove between said fourth and fifthcircumferential lands, and said means for effecting communicationbetween said line pressure inlet and outlet ports including the groovebetween said fourth and fifth circumferential lands, said means forblocking communication between said signal pressure inlet and outletports includes said fourth circumferential land.
 5. A hydraulic controlsystem as claimed in claim 4, wherein said first shift valve includes athird signal pressure port receiving said first signal pressure, saidvalve element of said first shift valve includes a fourth pressureacting area exposed to said first signal pressure from said third signalpressure port when said valve element of said first shift valve is inthe downshift position.
 6. A hydraulic control system as claimed inclaim 5, wherein said valve spool of said first shift valve includes asixth land larger in diameter than said fifth land, said valve elementof said first shift valve including a groove between said fifth andsixth lands, said fourth pressure acting area including the groovebetween said fifth and sixth lands, said sixth land of said valve spoolof said first shift valve blocking said third signal pressure port ofsaid first shift valve when said valve element of said first shift valveis in the upshift position.
 7. A hydraulic control system as claimed inclaim 1, further comprising:kickdown pressure generating means fordelivering a kickdown pressure to said second shift valve, wherein saidsecond shift valve includes a kickdown pressure port, said kickdownpressure port being selectively communicating with said signal pressureoutlet port of said second shift valve; wherein said valve element ofsaid second shift valve includes means for effecting communicationbetween said kickdown pressure port and said signal pressure outlet portof said second shift valve when said valve element of said second shiftvalve is in the second position to, in turn, effect communicationbetween said kickdown pressure generating means and said first signalpressure port of said first shift valve; and wherein said valve elementof said second shift valve includes means for blocking communicationbetween said kickdown pressure port and said signal pressure outlet portof said second shift valve when said valve element of said second shiftvalve is in the first position to, in turn, block communication betweensaid kickdown pressure generating means and said first signal pressureport of said first shift valve.
 8. A hydraulic control system as claimedin claim 7, wherein said valve element of said second shift valveincludes a third pressure acting area exposed to kickdown pressure fromsaid kickdown pressure port when said valve element of said second shiftvalve is in the second position.
 9. A hydraulic control system asclaimed in claim 8, wherein said valve element of said second shiftvalve is a valve spool having thereon first, second and thirdcircumferential lands which are larger in diameter in this sequence andwhich are axially spaced from each other, said valve element of saidsecond shift valve including at least one circumferential groove betweeneach two adjacent lands;wherein said first pressure acting area of saidvalve element of said second shift valve includes a groove between saidfirst and second lands of said valve spool of said second shift valveand a groove between said second and third lands of said valve spool ofsaid second shift valve; wherein said second pressure acting area ofsaid valve element of said second shift valve includes the groovebetween said second and third lands of said valve spool of said secondshift valve; and wherein said third pressure acting area includes thegroove between said first and second lands of said valve spool of saidsecond shift valve.
 10. A hydraulic control system as claimed in claim9, wherein said means for effecting communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve includes the groove between said first and second circumferentiallands of said valve spool of said second shift valve; andwherein saidmeans for blocking communication between said kickdown pressure port andsaid signal pressure outlet port of said second shift valve includessaid first circumferential land of said valve spool of said second shiftvalve.
 11. A hydraulic control system as claimed in claim 2, furthercomprising:kickdown pressure generating means for delivering a kickdownpressure to said second shift valve, wherein said second shift valveincludes a kickdown pressure port, said kickdown pressure port beingselectively communicating with said signal pressure outlet port of saidsecond shift valve; wherein said valve element of said second shiftvalve includes means for effecting communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in thesecond position to, in turn, effect communication between said kickdownpressure generating means and said first signal pressure port of saidfirst shift valve; and wherein said valve element of said second shiftvalve includes means for blocking communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in the firstposition to, in turn, block communication between said kickdown pressuregenerating means and said first signal pressure port of said first shiftvalve.
 12. A hydraulic control system as claimed in claim 3, furthercomprising:kickdown pressure generating means for delivering a kickdownpressure to said second shift valve, wherein said second shift valveincludes a kickdown pressure port, said kickdown pressure port beingselectively communicating with said signal pressure outlet port of saidsecond shift valve; wherein said valve element of said second shiftvalve includes means for effecting communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in thesecond position to, in turn, effect communication between said kickdownpressure generating means and said first signal pressure port of saidfirst shift valve; and wherein said valve element of said second shiftvalve includes means for blocking communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in the firstposition to, in turn, block communication between said kickdown pressuregenerating means and said first signal pressure port of said first shiftvalve.
 13. A hydraulic control system as claimed in claim 4, furthercomprising:kickdown pressure generating means for delivering a kickdownpressure to said second shift valve, wherein said second shift valveincludes a kickdown pressure port, said kickdown pressure port beingselectively communicating with said signal pressure outlet port of saidsecond shift valve; wherein said valve element of said second shiftvalve includes means for effecting communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in thesecond position to, in turn, effect communication between said kickdownpressure generating means and said first signal pressure port of saidfirst shift valve; and wherein said valve element of said second shiftvalve includes means for blocking communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in the firstposition to, in turn, block communication between said kickdown pressuregenerating means and said first signal pressure port of said first shiftvalve.
 14. A hydraulic control system as claimed in claim 5, furthercomprising:kickdown pressure generating means for delivering a kickdownpressure to said second shift valve, wherein said second shift valveincludes a kickdown pressure port, said kickdown pressure port beingselectively communicating with said signal pressure outlet port of saidsecond shift valve; wherein said valve element of said second shiftvalve includes means for effecting communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in thesecond position to, in turn, effect communication betwen said kickdownpressure generating means and said first signal pressure port of saidfirst shift valve; and wherein said valve element of said second shiftvalve includes means for blocking communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in the firstposition to, in turn, block communication between said kickdown pressuregenerating means and said first signal pressure port of said first shiftvalve.
 15. A hydraulic control system as claimed in claim 6, furthercomprising:kickdown pressure generating means for delivering a kickdownpressure to said second shift valve, wherein said second shift valveincludes a kickdown pressure port, said kickdown pressure port beingselectively communicating with said signal pressure outlet port of saidsecond shift valve; wherein said valve element of said second shiftvalve includes means for effecting communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in thesecond position to, in turn, effect communication between said kickdownpressure generating means and said first signal pressure port of saidfirst shift valve; and wherein said valve element of said second shiftvalve includes means for blocking communication between said kickdownpressure port and said signal pressure outlet port of said second shiftvalve when said valve element of said second shift valve is in the firstposition to, in turn, block communication between said kickdown pressuregenerating means and said first signal pressure port of said first shiftvalve.